The history of halftoning dates back to the seventeenth century print making technology of Mezzotint, which has evolved into classical screens, modern error diffusion and blue noise methods. Modern halftoning, commonly referred to as digital halftoning, encompasses a number of encoding methodologies often used to reduce the number of quantization levels per pixel in a digital image while maintaining the gray appearance of the image at normal reading distance. Halftoning techniques are widely employed in the printing and display of digital images. The need for halftoning arises either because the physical processes involved are binary in nature or the processes have been restricted to binary operation for reasons of cost, speed, memory or stability in the presence of process fluctuations. Examples of such processes include printing presses, ink jet printers, binary cathode ray tubes (“CRT”) displays and laser xerography.
Image generation devices often provide image information as eight bit signals representing an image pixel, although the number of bits in a signal may be lesser or greater. A pixel, also known as a picture element, is generally understood to be the smallest unit (e.g., a bit in a binary image, or a byte in an image with 8 bits per pixel) of an image that a particular image generation device can produce, store, or transmit. The eight bit signals mentioned above may represent for each pixel 256 distinct levels of color (i.e., 0–255 bit combinations), or in the case of a black and white image, gray scale levels. As alluded to above, many image output devices, such as digital color printers, monochrome facsimile devices or raster image processors, are capable of reliably producing only binary pixels on a printing medium, analogous to a “0” or “1” in the computer arts.
Groupings of these binary pixels, typically referred to as halftone dots, are arranged preferentially in a predetermined pattern within a digital halftone cell. For example, in order to create the illusion of a variety of output gray scale levels representative of the colors in an input image, the binary pixels are output at various counts per unit area (i.e., halftone cell). Assuming a “1-state” pixel is dark, the lower the count of 1-state output binary pixels per unit area, the lighter the tone will appear at normal reading distance. On the other hand, the higher the count of 1-state output binary pixels per unit area, the darker the tone will appear.
Some high quality digital color printers use halftone cells capable of reproducing 150 or more levels per color, which are approximately uniformly distributed in a reflectance space, preferentially a reflectance space that is perceptually uniform, such as L* in CIELab space as used in color science. In some printing technologies, several of the colors have a steep gamma, reaching approximately 10% reflectance (i.e., 90% darkness) while only approximately 50% of the input bits are turned on. Empirical testing has shown that a steep gamma combined with the vagaries of human eye perception may hinder the desired appearance of a gradual shading change over the range of the output device (e.g., printer). Moreover, printer rendition and environmental factors such as humidity or temperature may increase the chances of obtaining an undesired steep gamma during printing to further deteriorate print quality.